Background: Know: Recognize:

Anticipates: Fault management, Configuration management, MIB, ping, traceroute, special treatment for ICMP




How traceroute works
ICMP may be blocked
ICMP may be capped or expedited

Internet Control Message Protocol (ICMP)Edit

Note that ICMP messages are like TCP and UDP segments in that they are carried within IP datagrams. Although ICMP is used to be considered as a protocol above the IP protocol, however, it is closely tied to the IP protocol.
The Internet Protocol (IP) is one of the major components of network layer of the Internet. Another main component is Internet Control Message Protocol (ICMP) (specified in RFC 792), which is used for the communication between hosts and hosts or hosts and gateways (such as routers) for control purposes. The typical usage of ICMP is to report the problems or errors in IP datagram processing. For example, the inbound gateway of the destination host might send a "Destination host unreachable" ICMP message that indicates that host is not reachable.

The augment of IP by ICMP

The ICMP could augment IP by providing:

  • Error reporting

e.g. Destination unreachable message

An instance of this error is that according to the gateway’s routing table, if the distance to the destination network or host is infinity, that gateway might send back a "destination unreachable" ICMP message to the source host. A description of "Destination unreachable message" is indicated in page 4, RFC 792. Furthermore, the destination unreachable message is used as a criterion of traceroute stopping in xxHC How traceroute works.

  • Configuration information

e.g. ICMP router discovery message

The ICMP router discovery message, which is specified in RFC 1265, is used for helping hosts to discover the operational router in the subnet.

  • Testing facilities

e.g. Echo request or Echo reply message

The echo request/reply messages are used to test a specified remote host. The most well-known network administration utility which utilizes ICMP echo request/replay messages is Ping .

Used in Network Management tools

The ICMP message has been employed in such network management tools:

  • Ping
  • Traceroute

Ping is a network management tool for determining if there is a path between two hosts, whereas traceroute shows this actual path. Further information is covered in Ping and How traceroute works .

The standards-based MIB modules for ping and traceroute could be found in RFC 4560.

Note that the ICMP message might be blocked by the NetOps because of the security issue. One typical security issue is ICMP flood, which is one method of denial-of-service (DoS) attack. In this attack, the server might be overwhelmed by too many ICMP messages (such as ping) and then has not enough bandwidth to serve other users. More limitations of ICMP in network management are pointed out in ICMP may be capped or expedited and ICMP may be blocked .


See alsoEdit

Corresponding TELE9752 lecture slide


James F. Kurose Keith W. Ross , Computer Networking, a top-down approach, fifth edition, 2010, pp. 389-391. ISBN 0-13-136548-7.